The invention relates to a factory scale process for removing trace solvent from a material, especially a hygroscopic material. In particular, the invention relates to an improved process for producing suramin, the hexasodium salt of a polysulfonated naphthylurea, 8,8xe2x80x2-[carbonylbis[imino-3,1-phenylene-carbonylimino (4-methyl-3,1 -phenylene) carbonyl-imino]]bis-1,3,5-naphthalenetrisulfonic acid.
Suramin has been utilized clinically since the 1920s as an antiparasitic agent, and more recently has been shown to be active in the treatment of metastatic cancer; Stein et al., J. Clin. Oncology 1989;7(4):499-508. U.S. Pat. Nos. 5,767,110 and 5,597,830 describe the synergistic effect of suramin in combination with a vinca alkaloid or estramustine for treating cancer.
In addition, suramin can be used for treating autoimmune and allergic diseases such as rheumatoid arthritis, Crohn""s disease, ulcerative colitis, glomerular nephrites, systemic lupus erythematosus, polymyositis, Sjogren""s syndrome, asthma, and other inflammatory alveolar diseases. Suramin can also be used for treating AIDS and human T cell leukaemia virus.
The process for producing suramin is well-known, GB 224.849; however, it was unexpectedly found that on scale up to a commercial factory scale the drying regime of the final product was slow and difficult to optimize so as to obtain the desired finished product. It was found that unacceptable levels of reaction solvents were present in the final product. Similar problems arise in drying other materials, especially hygroscopic materials.
The object of the present invention is therefore to provide an improved process for removing trace solvents from a material, especially a hygroscopic material on a factory scale, which routinely and consistently produces material of a high quality.
According to the invention there is provided a factory scale process for removing trace solvent from a material comprising the steps of:
(a) providing a dryer;
(b) drying an isolated product in a dryer to remove free solvent;
(c) rewetting the product with water; and
(d) redrying the product to remove residual solvent to the required level.
The process of the invention facilitates the factory scale production of dried product with a very low level of residual solvent. Intermediate milling of the product is not required and drying time is much reduced.
Preferably, the product is rewetted with water vapor. Ideally, the added water is generated from pyrogen controlled deionized water.
In one embodiment of the invention the product is heated to facilitate the rewetting process.
In one embodiment of the invention the product is dried in a vacuum agitated pan dryer. In this case, preferably, the product is rewetted by injecting water vapor into the pan dryer. In a preferred embodiment the process includes the step of intermittently stopping the injection of water vapor into the pan dryer and agitating and sampling the product before recommencing the injection of water vapor. Ideally, the water vapor is injected into the pan dryer using an inert carrier such as nitrogen gas.
In another embodiment the product is dried in a tray dryer. In this case the process includes the steps of:
(a) loading at least one tray containing product into the tray dryer;
(b) loading at least one other tray containing water into the tray dryer; and
(c) heating the water in the dryer to rewet the product.
Preferably, the tray dryer includes a number of dryer cells and a tray containing product and a tray containing water are loaded into a dryer cell. In a preferred embodiment the process includes the steps of removing the water tray prior to drying of the product.
The invention is particularly applicable to hygroscopic materials such as suramin. The term hygroscopic refers to a product which takes up at least some water, for example, by absorption.
The invention also provides for a product whenever produced by a process of the invention. A further embodiment provides suramin whenever produced by a process of the invention.
Suramin has the following chemical structure: 
It is a white or slightly pink or cream-coloured powder. It has a slightly bitter taste. It is hygroscopic and is freely soluble in water and in physiological saline.
Suramin is produced from 8-[3-(3-aminobenzamido)-4-methylbenzamido]-1,3,5-napthalenetrisulfonic acid sodium salt in the presence of bistrichloromethylcarbonate (BTCMC)(Triphosgene) and sodium hydroxide as shown in the following reaction scheme. 
Typical methods for drying the final product involve drying under vacuum for up to 8 days. During the drying procedure the drying is interrupted and the product is milled and sieved before recommencing drying. However, it was found that when this process was scaled up to a commercial factory scale the process resulted in a product with high levels of reaction solvents, such as methanol, still present.
The invention provides an improved drying regime using a vacuum agitated pan dryer or tray dryer. The initial drying procedure reduces the methanol content to 0.3% to 0.8%. The product is then rewetted by passing a stream of steam through the headspace of the dryer. When the water content of the product has increased sufficiently, to 10% to 15%, rewetting is stopped, and the product is redried under vacuum at 80xc2x0 C. until dry. This procedure allows high quality suramin with a water content of less than 10% and a methanol content of less than 0.1% to be produced on a consistent basis. Intermediate milling of the material is not necessary and the drying time is much reduced.
The invention will be more clearly understood from the following description given by way of example.